The present invention is directed to battery separators that incorporate sodium sulfate to reduce hydration shorts.
Storage batteries are generally composed of at least one pair of electrodes of opposite polarity, usually a series of adjacent electrodes of alternating polarity, and an electrolyte. The current flow between these electrodes is maintained by an electrolyte which may be acid, alkaline, or substantially neutral, depending upon the nature of the battery system. Separators are positioned in batteries between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates while freely permitting electrolyte movement and ionic transfer. Separator components have taken many forms. In modern battery design, the separator is in the form of a sheet or film or more preferably, a thin envelope surrounding each electrode plate of one polarity.
One of the critical elements in a battery design is the separator component, which should have a combination of properties. The battery separator must be resistant to degradation and instability with respect to the battery environment, including the other battery components and the battery chemistry. Thus, the battery separator must be capable of withstanding degradation of strong acids (such as sulfuric acid commonly used in acid battery designs) or strong alkali (such as potassium hydroxide commonly used in alkaline battery designs) and to do so under ambient and elevated temperature conditions. Further, the separator should be of highly porous character to provide a battery of high energy density. Although battery separators of thick or heavy design have been utilized in the past, such materials detract from the overall energy density of the battery by reducing the amount of electrodes and/or electrolyte that can be contained in a predetermined battery configuration and size. Another criterium is that the battery separator must be capable of allowing a high degree of electrolyte movement. Stated differently, an effective separator membrane must exhibit a low electrical resistance when in the battery. The lower the electrical resistance, the better the overall battery performance will be. A still further criterium is that the separator should be capable of inhibiting formation and growth of dendrites. Such dendrite formation occurs during battery operation when part of the electrode material becomes dissolved in the electrolyte and, while passing through the separator, deposits therein to develop a formation which can, after a period of time, bridge the thickness of the separator membrane and cause shorting between electrodes of opposite polarity.
Various microporous membranes or sheet materials have been suggested for utilization as a battery separator. Separators conventionally used in present battery systems are formed of polymeric films which when placed in an electrolyte or an electrolyte system, are capable of exhibiting a high degree of conductivity while being stable to the environment presented by the battery system. The films include macroporous as well as microporous materials. The porosity permits transportation of the electrolyte. Examples of such separators include polyolefin sheets which have been stretched and annealed to provide microporosity to the sheet, such as is described in U.S. Pat. Nos. 3,558,764, 3,679,538 and 3,853,601. U.S. Pat. No. 3,351,495 to Larsen et al. discloses a battery separator having a relatively low pore size and satisfactory electrical resistance characteristics made from a high molecular weight polyolefin having an average molecular weight of at least 300,000, a standard load melt index of substantially zero, and a reduced viscosity of not less than 4. The separator is manufactured by extruding the high molecular weight polyolefin in admixture with an inert filler and a plasticizer and then extracting the plasticizer by the use of a suitable solvent. Other conventional separators for lead-acid and gas recombination batteries contain mostly glass fibers, and in particular, asbestos glass fibers. In view of the recent scrutiny to which asbestos has been subjected, it would be desirable to provide a non-asbestos containing battery separator that exhibits the same or better characteristics than the conventional asbestos-containing separators. Moreover, polymer separators have higher tensile strength than glass mat separators and thus are more conducive to high speed manufacturing, which can significantly reduce production costs.
One problem associated with various batteries is hydration shorts. When a battery is discharged slowly, which occurs for example when the headlights of an automobile are left on for an extended period of time after the engine has been turned off, water is formed which dilutes the electrolyte. As more and more water is formed, a decrease in the specific gravity of the electrolyte occurs, and lead sulfate tends to precipitate onto the battery plates and battery separator (which manifests itself in visible white spots). The precipitated lead sulfate closes the pores in the separator. Although such a battery may be capable of maintaining a charge after being recharged, several of these "slow discharge" episodes will result in shortened battery life. Higher than normal ratios of active material to electrolyte are also a factor in causing hydration problems.
One conventional way of addressing this phenomenon is the addition of sodium sulfate tablets to the electrolyte in a post-manufacturing step. In theory, any water formed during slow battery discharge will cause a portion of the sodium sulfate tablets to dissolve in the electrolyte, thereby maintaining the specific gravity of the electrolyte within the desired range. However, this method is uneconomical from a production standpoint.
It is therefore an object of the present invention to develop a battery separator that will reduce and/or eliminate battery performance problems related to hydration shorts.